The present invention relates to a selector mechanism for a continuously variable belt-drive automatic transmission for a vehicle, and particularly to a mechanism for performing the selection of operational ranges of the automatic transmission.
A known continuously variable belt-drive transmission (called hereinafter CVT) comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. Further, a selector mechanism is provided for selecting operational ranges such as a drive range (D), high engine speed drive range (Ds), reverse range (R), neutral range (N), and parking range (P). The selection of these ranges are performed by manipulating a selector lever. Thus, a driving position (D range) and a reverse driving position (R range), neutral position and parking position are provided for the selector lever.
European Patent Application EP-A-0 151 024 discloses a CVT in which a selector mechanism having a synchromesh mechanism is provided between an electromagnetic clutch and an input shaft of a belt and pulley device. The selector mechanism has a manually operated selector lever mechanically connected thereto through a link mechanism.
In order to reduce the shifting effort, there has been proposed a selector mechanism operated by a hydraulic actuator which is connected to the selector lever. When the selector lever is operated, the actuator operates to shift a part of the synchromesh mechanism such as a fork or sleeve to engage the synchromesh mechanism. In such a system, the selector lever can be operated regardless of the engagement of the synchromesh mechanism.
Generally, the CVT is so arranged that when the selector lever is shifted from the neutral position to the driving position and to the reverse position, drag current flows in a magnetizing coil of the electromagnetic clutch to apply drag torque to the drive pulley to remove clearances in the transmission.
On the other hand, it may occur that, when shifting the selector lever, time elapses before the sleeve of the synchromesh mechanism engages with a corresponding gear in synchronism with it. Accordingly, it may happen that the electromagnetic clutch engages before the engagement of the synchromesh mechanism. When the electromagnetic clutch engages, the drag torque is applied. Therefore, it is difficult to engage the synchromesh mechanism. Further, the selector lever may be shifted to the neutral position during driving of the vehicle and shifted again to the driving position. In such a case, the actuator fails to engage the synchromesh mechanism, because of a large difference between the speeds of the sleeve and the corresponding gear.